Metallic coatings, comprising alloys of iron, nickel, and/or cobalt, are used on a wide variety of industrial hardware in order to provide properties, such as wear resistance, abrasion resistance, corrosion resistance, and lubricity, which are lacking in the component substrate material of the hardware. The metallic coating layer may be formed by modifying the surface layer of a metallic substrate by a diffusion process, such as chromizing. Alternatively, the metallic coating may be formed by depositing a distinct coating layer or layers onto the component substrate surface, forming what is referred to as a metallic overlay coating.
The metallic coatings may include dispersed phases, such as carbides, borides, oxides, and/or silicides, within the iron, nickel, and/or cobalt alloy matrix to enhance the performance of the coating. Examples of metallic wear-resistant coatings are chrome-carbide/nickel-chrome and tungsten carbide/cobalt coatings, which are used to provide wear and abrasion resistance at critical locations on gas turbine components such as fan blade mid-spans and turbine seal areas. A variety of metallic overlay coatings are disclosed in U.S. Pat. Nos. 4,588,606, 4,666,733, 4,803,045, 5,326,645, and 5,395,221, which patents are incorporated herein by reference.
One important class of metallic overlay coatings is known as an "MCrAlY" coating, in which M is Ni, Co, and/or Fe. These MCrAlY coatings are typically applied by physical vapor or thermal spray deposition and provide high temperature oxidation and/or corrosion resistance. Examples of MCrAlY coatings are disclosed in U.S. Pat. Nos. 3,993,454, 4,585,481, and European patents EP 0688885 and EP 0688886, each of which is incorporated herein by reference.
Metallic overlay coatings may be used as an intermediate layer to bond a subsequent ceramic coating to a metallic substrate. Examples of overlay coatings used as bondcoats are disclosed in U.S. Pat. Nos. 5,520,516, 5,536,022, 4,861,618, 5,384,200, 5,305,726, 5,413,871, and 5,498,484, each of which is incorporated herein by reference.
Metallic MCrAlY overlay coatings are commonly utilized for oxidation and corrosion protection of high temperature, high strength cobalt and nickel superalloy gas turbine engine components. These components are usually complex castings with intricate internal passages which provide cooling to the component and allow the component to operate in turbine environment where the gas temperature may exceed the melting temperature of the superalloy. The demands for more efficient cooling and lower weight results in strict dimensional specifications for component wall thickness and coating thickness and uniformity. For example, there are regions on small, intricate aircraft gas turbine airfoils where the actual thickness of the part may be as thin as 1-2 mm. For these components, the MCrAlY coating thickness specification may be on the order of 50-75 .mu.m. Large industrial ground turbine (IGT) blades and vanes also are fabricated to provide internal cooling and also have strict dimensional tolerances on component wall thickness in order to satisfy component strength requirements. For these components the MCrAlY coating thickness requirements are typically on the order of 150-200 .mu.m. The MCrAlY coatings provide oxidation and corrosion protection by formation of a protective aluminum oxide scale which forms at high temperature during service. The aluminum in the coatings, typically on the order of 6-18 percent, provides a reservoir for aluminum oxide scale reformation as degradation occurs due to thermal cycling, erosion, corrosion, etc. Because the temperature, erosion activity, and deposition of foreign contaminants varies from area to area, degradation often occurs locally, resulting in significant differences in coating thickness and chemistry over the surface of a part with continued service exposure. The coating chemistry can also change due to diffusion between the coating and the substrate. The interdiffusion between coating and substrate is also a function of temperature and so compositional changes due to interdiffusion will also vary from region to region a part.
As the strength and lifetime requirements for industrial components, especially those exposed to high operational temperatures, have increased, processing complexity and the cost of these components has greatly increased. It is, therefore, important that the components protected by these coatings be re-used, that is, taken from service at regular intervals and processed where possible to restore materials dimensions and properties and be returned into service. This processing usually requires the removal of the overlying protective coatings.
As was mentioned, a major obstacle in the removal of these coatings is that the coatings are often degraded, and have local variations in thickness, due to accelerated local wear, oxidation, corrosion, or erosion. Thus, a part which had a coating with an applied thickness varying between 150 and 200 .mu.m may be returned for repair with some regions having coating thicknesses of less than 50 .mu.m whereas other regions have virtually the original coating thickness of 200 .mu.m. Additionally, the coating chemistry may also vary across the surface of a part due to local variations in exposure to temperatures and contaminants. These local variations in thickness and chemistry complicate coating removal by affecting local coating removal rates. In addition, while removing the coatings, it is imperative that damage to the underlying substrate material, or removal of substrate material itself, be minimized. Attack or removal of the substrate below the degraded coating can cause component loss due to thinning of the component wall.
One present method for removal of metallic overlay coatings is by utilizing strip solutions of nitric or hydrochloric acid which attack the aluminum-rich phases in the coating. However, these acid strip solutions are ineffective for removing metallic overlay coatings in which the aluminum content has been reduced by diffusion and dilution into the base material and by repeated thermal cycling. Moreover, because the loss of aluminum from the coating frequently varies in severity over the surface of the coating, acid stripping can cause non-uniform stripping rates and possibly attack of the base material substrate itself. Attack of the substrate can result in component loss due to local thinning or degradation of the component wall thickness which ultimately renders the component unusable due to insufficient wall thickness.
Metallic overlay coatings which cannot be successfully stripped with acid solutions are often removed by manual mechanical means, such as by grinding, belt sanding or intense blasting with abrasive media and/or water at high pressure. These mechanical means are difficult to control and may cause loss of the dimensional integrity of the substrate component.
Several recent methods to prepare coated turbine blades for stripping include aluminizing the blades by pack cementation prior to stripping to make the coating easier to remove by chemical and/or mechanical means. In an article entitled "Refurbishment Procedures for Stationary Gas Turbine Blades", Proceedings of an International Conference jointly sponsored by ASM International and The Electric Power Research Institute, Phoenix, Ariz. (April 17-19, 1990), edited by Viswanathan and Allen, Burgel et al. disclose what they refer to as "one negative example" of what can occur during stripping using this approach. Burgel et al. disclose that, because pack cementation requires high temperatures which lead to inward diffusion of elements of the residual coating into the microstructure of the turbine blade, the aluminizing procedure results in deterioration of the whole wall thickness at the leading edge of the blade.
Czech and Kempster, PCT Application WO 93/03201 (1993), disclose a pack cementation aluminizing procedure which purportedly overcomes the problems associated with aluminizing disclosed by Burgel et al. by ensuring that all corrosion products in the coating and substrate are completely enclosed within the deposited aluminide coating. In the procedure of Czech, the surface of a superalloy or steel part is first cleaned, by chemical or physical means, to remove a substantial part of corrosion products on the surface. The cleaned part is then aluminized in an inert atmosphere by either pack aluminizing, out of pack aluminizing, or gas phase aluminizing to a depth that encloses all products of corrosion, including deep corrosion products, thus preventing the inward diffusion of deleterious phases, such as sulfides, within the substrate. In order to achieve a depth of aluminization that encloses all products of corrosion, high processing temperatures of at least 1050.degree. C. must be used. The procedure of Czech results in an aluminide layer of uniform thickness greater than 150 .mu.m over the surface of the substrate.
The procedure of Czech has several disadvantages which add process complexity or limit its applicability. Because all corrosion products, including "grain boundary sulfides", must be encompassed during the aluminization process, which requires a depth of aluminization of greater than 150 .mu.m, temperatures of 1050.degree. C. or higher must be employed, either in an initial treatment if a low activity pack is used or as a subsequent treatment if a high activity pack is used initially. These high temperatures can cause damage to delicate metal parts, such as turbine blades. These high temperatures also can complicate the removal of the aluminide layer in many applications. Processing aluminide layers in temperature ranges above 1050.degree. C. on carbon-containing cast nickel and cobalt superalloy materials produces a zone of carbide precipitates below a diffused aluminide surface layer. The mechanisms and reasons for the formation of this "carbide zone" are well established within the technical literature related to formation of aluminide layers on gas turbine alloy materials (see by reference, "Formation and Degradation of Aluminide Coatings on Nickel-Base Superalloys, Goward et al. Transactions of the ASM, Vol. 60, 1967, pages 228-241). Formation of this zone of carbide precipitates during aluminization complicates removal of the aluminide layer, because the zone containing these carbide precipitates is difficult to remove by mechanical means and typically requires a combination of chemical and mechanical methods to completely remove it and expose superalloy base metal surface. Czech reports that he prefers a combination of mechanical and chemical methods for removing the aluminide layer.
Also, the method of Czech, utilizing pack cementation, results in the surface of the part receiving the entire depth of the aluminizing treatment unless the surface of the part is masked to completely block the formation of any aluminide layer at all in the masked area. Thus, the method of Czech does not permit controlled formation of aluminide layers of varying depths at different regions of the surface of a part, such non-uniform aluminide layers being desirable when a coating to be removed has a non-uniform thickness or when corrosion depth varies locally within a metallic surface layer.
Further, because of the necessity of forming an aluminide layer which encloses all corrosion products to a depth of 150 .mu.m, the method of Czech precludes a partial strip process of a coating which has corrosion, wear, or oxidation damage confined to a relatively thin outer surface layer of the coating, with the bulk of the underlying coating being suitable for re-use or re-coating. For example, as disclosed by Czech, a part having a 100 .mu.m thick coating with corrosion limited to the outer 50 .mu.m of the surface would have the entire coating and a portion of the underlying substrate material aluminized and removed.
An additional disadvantage of the method of Czech is that, because of the nature of the pack cementation process, an inert atmosphere must be used to protect aluminum and other components in the pack from high-temperature attack by atmospheric oxygen.
Guerreschi EP 0713957 A1 discloses a method for localized aluminization of an MCrAlY coated turbine blade which method comprises cleaning the blade by sand blasting, masking off with tape those areas which are to be left unaluminized, applying a layer of aluminum by plasma spray, and heating the blade to the solution heat treat temperature of the blade substrate, which temperatures are generally above 1100.degree. C., in a furnace and in an inert atmosphere. The treatment of Guerreschi causes the aluminum to diffuse into the coating, which produces a brittle aluminide coating which can be subsequently removed by sand blasting.
The method of Guerreschi has the disadvantages that high temperature treatment is required, above the solution heat treat temperature of the metal substrate, which temperatures can lead to thermal damage of delicate metal parts, such as those in turbomachinery, and can cause the formation of undesirable carbide phases within a carbon-containing superalloy substrate. Furthermore, during subsequent heating, the plasma spray deposited aluminum layer tends to flow laterally due to surface tension and gravitational forces, with resultant undesired removal of base material from masked-off regions and unintended differences in depth of aluminization and surface layer removal. See FIGS. 1 and 2.
The method of the present invention overcomes the disadvantages of the prior art in providing a method for the removal of metallic coatings which method comprises low temperature application of an aluminide layer by slurry deposition on the metallic surface. The method of the invention obviates the need to encompass all products of corrosion, can be precisely varied in thickness across the surface to be treated, can be applied locally with precision, may be performed in a non-inert atmosphere, and does not result in undesirable phase transformations within the substrate.